the question isn't that sensible in it self as all PCs need a hard drive to operate so there is no real choice. If you mean where to store the music, you have to get an external hard drive anyway because 1) u can transfer music to other computers far easier 2) if your laptop doesn't turn on you cant access your music easy. Otherwise by nature internal drives are faster by design....usb connections were not originally designed for hard drive access.
Internal less worries...also also have a backup external bootable drive...superduper program is excellent for this it makes a exact bootable copy of your internal hard drive..
internal ssd but never get a enclosure/external because it lowers the transfer speed or quality of the os thats downgraded through usb 2.0/3.0
Internal definitely ... unless you have USB 3.0 and better yet ...Samsung SSDs ....lightning speed read/write performance
External for sure if you want to move portable and freely with your data. Plus, how big is a standard WD Hard drive anyways, right? And you can find some pretty cheap ones that are like 2TB+
Secondary HD Internal - Too often I see external man's dem hard drive getting pulled out when a next dj comes to set up or leave.. Then you have to wait for it to reload.. Place quiet quiet.. MC talkin shit for 5 mins.. Nah man.. Internal ALL THE WAY SON!
Preferable an external hard drive, because you can take it very easy with you if you have alot of holiday pics.
I personally find external harddrives better, because mostly I am using my files on different systems